Yeast amino acid permeases are responsible for transporting their substrates into the cell in response to the external environment and the nutritional: requirements of the cell. A family of yeast amino acid/auxin permeases has been identified through the Yeast Genome Project and the substrate range and regulation of many of these permeases has been elucidated. Through the study of these permeases, increasing evidence has accumulated that there are groups of permeases with distinct, and limited, metabolic roles in the cell. For example, it appears that these permeases can be classified as anabolic or catabolic in function and that anabolic specific permeases are not capable of supplying their amino acid substrate for growth. The mechanism of differentiation of function of these highly homologous permeases is not understood at present. Two possible mechanisms are presented by which an anabolic transporter could provide an amino acid for protein synthesis but not for growth: signaling or channeling. The experiments outlined in this proposal are designed to elucidate whether an amino acid permease can form part of the signaling pathway that indicates the availability of amino acid as well as transporting it into the cell. We will also investigate whether different types of permease contribute their substrate to the free amino acid pools of the cell or whether certain transporters 'channel' their substrate into a particular metabolic fate. These studies will be initiated focusing on tryptophan and its anabolic permease, Tat2. Tryptophan uptake is of particular interest because there are many unusual phenotypes associated with low tryptophan levels in yeast. There are also interesting parallels between yeast tryptophan uptake and auxin response in plants.